Avoid preview-image delaying the switching of the top bar title (mobile views)

Use Intersection Observer API to optimize the performance of title switching

_javascript/commons/topbar-switch.js

@@ -4,6 +4,7 @@
 
 $(function() {
   const $topbarWrapper = $("#topbar-wrapper");
+  const $topbarTitle = $("#topbar-title");
   const $panel = $("#panel-wrapper");
   const $searchInput = $("#search-input");
 
@@ -54,7 +55,7 @@ $(function() {
   }
 
   $(window).scroll(function(event) {
-    if ($("#topbar-title").is(":hidden")) {
+    if ($topbarTitle.is(":hidden")) {
       didScroll = true;
     }
   });

_javascript/commons/topbar-title.js

@@ -1,46 +1,61 @@
 /*
- * Top bar title auto change while scrolling in mobile screens.
+ * Top bar title auto change while scrolling up/down in mobile screens.
 */
 
 $(function() {
+  const titleSelector = "div.post>h1:first-of-type";
+  const $pageTitle = $(titleSelector);
+  const $topbarTitle = $("#topbar-title");
 
-  const topbarTitle = $("#topbar-title");
-  const postTitle = $("div.post>h1");
+  if ($pageTitle.length === 0 /* on Home page */
+      || $pageTitle.hasClass("dynamic-title")
+      || $topbarTitle.is(":hidden")) {/* not in mobile views */
+    return;
+  }
 
-  const DEFAULT = topbarTitle.text().trim();
-
-  let title = (postTitle.length > 0) ?
-    postTitle.text().trim() : $("h1").text().trim();
+  const defaultTitleText = $topbarTitle.text().trim();
+  let titleText = $pageTitle.text().trim();
+  let hasScrolled = false;
+  let lastScrollTop = 0;
 
   if ($("#page-category").length || $("#page-tag").length) {
     /* The title in Category or Tag page will be "<title> <count_of_posts>" */
-    if (/\s/.test(title)) {
-      title = title.replace(/[0-9]/g, "").trim();
+    if (/\s/.test(titleText)) {
+      titleText = titleText.replace(/[0-9]/g, "").trim();
     }
   }
 
-  /* Replace topbar title while scroll screens. */
-  $(window).scroll(function () {
-    if ($("#post-list").length /* in Home page */
-      || postTitle.is(":hidden") /* is tab pages */
-      || topbarTitle.is(":hidden") /* not mobile screens */
-      || $("#sidebar.sidebar-expand").length) { /* when the sidebar trigger is clicked */
-      return false;
+  let options = {
+    rootMargin: '-48px 0px 0px 0px', // 48px equals to the topbar height (3rem)
+    threshold: [0, 1]
+  };
+
+  let observer = new IntersectionObserver((entries) => {
+    if (!hasScrolled) {
+      hasScrolled = true;
+      return;
     }
 
-    if ($(this).scrollTop() >= 95) {
-      if (topbarTitle.text() !== title) {
-        topbarTitle.text(title);
+    let curScrollTop = $(window).scrollTop();
+    let isScrollDown = lastScrollTop < curScrollTop;
+    lastScrollTop = curScrollTop;
+    let heading = entries[0];
+
+    if (isScrollDown) {
+      if (heading.intersectionRatio === 0) {
+        $topbarTitle.text(titleText);
       }
     } else {
-      if (topbarTitle.text() !== DEFAULT) {
-        topbarTitle.text(DEFAULT);
+      if (heading.intersectionRatio === 1) {
+        $topbarTitle.text(defaultTitleText);
       }
     }
-  });
+  }, options);
 
-  /* Click title remove hover effect. */
-  topbarTitle.click(function() {
+  observer.observe(document.querySelector(titleSelector));
+
+  /* Click title will scroll to top */
+  $topbarTitle.click(function() {
     $("body,html").animate({scrollTop: 0}, 800);
   });